Atom column quant with set_elements_info

This change should assign the number of atoms in each column using the statistical_quant function. Currently it works fine for homogeneous structures. To deal with Atom Position with more than one element I guess a list of fractional z coordinates and/or bond lengths will be required.

I have left in the old design in which an Atom Lattice is created and displayed along with the Gaussian-fitted histogram because I feel it makes the quantification very clear to the user. Thoughts on this @TomSlater?

• Assign each Atom Position with the set_element_info method.
• Generalise for max_atom_nums > model.n_components.
• Change image param to optional.
• Add tests to verify:
  • correct number of atoms assigned.
  • correct fractional z coordinate is assigned.
• Minor fixes in statistical_quant and get_statistical_quant_criteria.
• z_ordering for control of how the atomic model is created during statistical_quant. In future, this should be its own function. Then, the users can flip between z_ordering on the fly.
• Some nice visualisations with ASE in the docstring and for the documentation walkthrough.